Polyolefins such as polyethylene and polypropylene may be prepared by particle form polymerization, also referred to as slurry polymerization. In this technique, feed materials such as monomer, catalyst and diluent are introduced to a polymerization reactor, for example, a loop reactor, and an intermediate product slurry containing solid polyolefin particles in a liquid medium is withdrawn or taken off.
In continuous loop reactors, the various feed materials may be introduced to the loop reaction zone in various ways. For example, the monomer and catalyst may be mixed with varying amounts of diluent prior to introduction to the reaction zone. In the loop reaction zone, the monomer and catalyst become dispersed in the fluid slurry. As they circulate through the loop reaction zone in the fluid slurry, the monomer reacts at the catalyst site in a polymerization reaction. The polymerization reaction yields solid polyolefin particles in the fluid slurry.
A catalyst is provided to the polymerization reactor to catalyze the polymerization process. In conventional polyethylene loop reaction processes, the dry, solid catalyst is combined with olefin-free diluent in an unagitated vessel known as a mud chamber. The catalyst settles within the diluent to form a catalyst mud. After the catalyst mud is prepared, it is then fed into a lead-in pipe by a ball-check feeder located at the bottom of the catalyst mud chamber. The lead-in pipe then feeds the catalyst mud (or catalyst slurry if it has been sufficiently agitated) to the loop reactor.
The ball-check feeder discharges the catalyst mud from the catalyst mud chamber in an intermittent fashion. The ball-check feeder includes a cylinder attached to a rotating cam. The cylinder has an open top end and contains a ball that slides up and down within the cylinder. In operation, the cylinder is upright with the top end under the catalyst mud chamber and the ball positioned near the top end. The catalyst mud chamber pours catalyst mud into the cylinder through the top end with such force that the ball is pushed down toward a bottom end of the cylinder, and the catalyst mud fills the cylinder above the ball. The cam arm then rotates the cylinder such that the top end of the cylinder faces downward and is aligned above the lead-in pipe. The catalyst mud then pours out of the cylinder into the lead-in pipe, and the ball falls back into position near the top end of the cylinder. The cam arm then rotates the cylinder back such that the top end of the cylinder is again beneath the catalyst mud chamber to receive more catalyst mud. Thus, the catalyst mud is fed into the loop reactor in a series of discrete loads.
The ball-check feeder and the catalyst mud chamber suffer from several drawbacks. First, the amount of catalyst mud delivered by the ball-check feeder may vary with each rotation. For example, the cylinder may be only half-filled with catalyst slurry from the catalyst mud chamber. At other times, the cylinder may receive mainly liquid diluent, with very little catalyst from the catalyst mud chamber. Also, if the ball-check feeder is not properly sealed, the pressure differential between the catalyst mud chamber and the reactor can cause catalyst to bypass the ball-check feeder and go into the reactor, which may lead to an excessive amount of catalyst being fed into the reactor. The lack of consistency in the delivery of the catalyst mud can make it difficult to determine how much catalyst is being fed into the loop reactor at any given time. Therefore, an operator using the ball-check feeder cannot, in the regular course of operation, accurately monitor the amount of catalyst being delivered to the reactor.
The catalyst feed rate may be inferred by the number of times the catalyst is dumped out of the ball-check feeder per unit of time. However, because of the inconsistency in the amounts of catalyst in each dump (as discussed above), this method can be inaccurate. Also, if the operators reduce or increase the amount of catalyst being fed into the reactor by changing the speed of the feeder, the change in the speed of the feeder is generally not reliably proportional to the catalyst feed rate. The catalyst feed rate may also be inferred by the amount of polymerization that takes place in the reactor. However, such a method occurs after too much or too little catalyst has already been fed into the reactor. Too much or too little catalyst mud fed into the loop reactor may adversely affect the polymerization process. Therefore, the inconsistency and unpredictability of the ball-check feeder and catalyst mud chamber increase the possibility that the polymerization process in the loop reactor will not be performed under desired conditions.
Another problem with the ball-check feeder is that the settled catalyst mud generally does not have a homogeneous catalyst-to-diluent weight ratio. Since the catalyst mud chamber does not agitate the catalyst slurry/diluent mixture, the catalyst settles within the mixture to form a layer of catalyst mud beneath a layer of diluent. This catalyst mud layer has a greater concentration of catalyst at the bottom than at the top. Therefore, the concentration of the catalyst mud fed into the reactor is greater when the mud chamber is first activated, and the concentration decreases as the mud chamber empties. Non-homogeneous catalyst slurry may lead to too much or too little catalyst being fed into the loop reactor, which again may adversely affect the polymerization process. Also, the ball-check feeder may leak catalyst slurry into the lead-in pipe between rotations, so extra catalyst slurry may at times be fed into the loop reactor.
Other apparatuses and processes have been developed to deliver catalyst to a polymerization reactor. However, there remains a desire for a system that continuously and reliably delivers catalyst slurry to a loop reactor.